In a conventional pen injector, such as the Owen Mumford Autopen®, a cartridge containing therapeutic material, for example insulin, is received in a cartridge holder which is connected to a pen body. A needle is typically removably attached to the cartridge holder for delivery of the therapeutic material. The pen body is provided with a mechanism arranged to expel successive single metered doses of therapeutic material from the cartridge via the needle. Such mechanisms generally comprise a plunger which is arranged to engage a piston of the cartridge and to move forward by a predetermined increment in response to a user pressing release button. It will be appreciate that pen injectors are arranged to administer a plurality of repeatable single metered doses and that typically the volume of each individual dose may be variable. Therefore, the mechanism generally further comprises a dose selector, for example a dial, to adjust the movement of the plunger.
It is desirable for pen injectors to be of a compact form so that they can be carried around and used unobtrusively. Further compact injectors may be simple to manufacture, assemble and use with consequent savings in manufacturing and assembly costs, and a lower environmental impact.